Irrigated ablation catheter having irrigation ports with reduced hydraulic resistance

ABSTRACT

An irrigated ablation catheter includes a tip electrode with a thin shell and a plug to provide a plenum chamber. The tip electrode has an inlet of a predetermined size and noncircular shape, and outlets in the form of fluid ports formed in the thin shell wall. The plurality of the fluid ports is predetermined, as is their diameter. Each fluid port has a tapered configuration, for example, a frustoconical configuration, with a smaller inlet diameter and a larger outlet diameter.

RELATED APPLICATIONS

This application is a continuation-in-part of and claims priority to andthe benefit of U.S. application Ser. No. 12/769,592, filed Apr. 28,2010, and U.S. application Ser. No. 12/770,582, filed Apr. 29, 2010, theentire contents of which are incorporated herein by reference.

FIELD OF INVENTION

The present invention relates to an electrophysiologic catheter that isparticularly useful for ablation and sensing electrical activity ofheart tissue.

BACKGROUND OF INVENTION

Cardiac arrhythmias, and atrial fibrillation in particular, persist ascommon and dangerous medical ailments, especially in the agingpopulation. In patients with normal sinus rhythm, the heart, which iscomprised of atrial, ventricular, and excitatory conduction tissue, iselectrically excited to beat in a synchronous, patterned fashion. Inpatients with cardiac arrythmias, abnormal regions of cardiac tissue donot follow the synchronous beating cycle associated with normallyconductive tissue as in patients with normal sinus rhythm. Instead, theabnormal regions of cardiac tissue aberrantly conduct to adjacenttissue, thereby disrupting the cardiac cycle into an asynchronouscardiac rhythm. Such abnormal conduction has been previously known tooccur at various regions of the heart, such as, for example, in theregion of the sino-atrial (SA) node, along the conduction pathways ofthe atrioventricular (AV) node and the Bundle of His, or in the cardiacmuscle tissue forming the walls of the ventricular and atrial cardiacchambers.

Cardiac arrhythmias, including atrial arrhythmias, may be of amultiwavelet reentrant type, characterized by multiple asynchronousloops of electrical impulses that are scattered about the atrial chamberand are often self propagating. Alternatively, or in addition to themultiwavelet reentrant type, cardiac arrhythmias may also have a focalorigin, such as when an isolated region of tissue in an atrium firesautonomously in a rapid, repetitive fashion. Ventricular tachycardia(V-tach or VT) is a tachycardia, or fast heart rhythm that originates inone of the ventricles of the heart. This is a potentiallylife-threatening arrhythmia because it may lead to ventricularfibrillation and sudden death.

Diagnosis and treatment of cardiac arrythmias include mapping theelectrical properties of heart tissue, especially the endocardium andthe heart volume, and selectively ablating cardiac tissue by applicationof energy. Such ablation can cease or modify the propagation of unwantedelectrical signals from one portion of the heart to another. Theablation process destroys the unwanted electrical pathways by formationof non-conducting lesions. Various energy delivery modalities have beendisclosed for forming lesions, and include use of microwave, laser andmore commonly, radiofrequency energies to create conduction blocks alongthe cardiac tissue wall. In a two-step procedure—mapping followed byablation—electrical activity at points within the heart is typicallysensed and measured by advancing a catheter containing one or moreelectrical sensors (or electrodes) into the heart, and acquiring data ata multiplicity of points. These data are then utilized to select theendocardial target areas at which ablation is to be performed.

Electrode catheters have been in common use in medical practice for manyyears. They are used to stimulate and map electrical activity in theheart and to ablate sites of aberrant electrical activity. In use, theelectrode catheter is inserted into a major vein or artery, e.g.,femoral artery, and then guided into the chamber of the heart ofconcern. A typical ablation procedure involves the insertion of acatheter having a tip electrode at its distal end into a heart chamber.A reference electrode is provided, generally taped to the skin of thepatient or by means of a second catheter that is positioned in or nearthe heart. RF (radio frequency) current is applied to the tip electrodeof the ablating catheter, and current flows through the media thatsurrounds it, i.e., blood and tissue, toward the reference electrode.The distribution of current depends on the amount of electrode surfacein contact with the tissue as compared to blood, which has a higherconductivity than the tissue. Heating of the tissue occurs due to itselectrical resistance. The tissue is heated sufficiently to causecellular destruction in the cardiac tissue resulting in formation of alesion within the cardiac tissue which is electrically non-conductive.During this process, heating of the electrode also occurs as a result ofconduction from the heated tissue to the electrode itself. If theelectrode temperature becomes sufficiently high, possibly above60.degree. C., a thin transparent coating of dehydrated blood proteincan form on the surface of the electrode. If the temperature continuesto rise, this dehydrated layer can become progressively thickerresulting in blood coagulation on the electrode surface. Becausedehydrated biological material has a higher electrical resistance thanendocardial tissue, impedance to the flow of electrical energy into thetissue also increases. If the impedance increases sufficiently, animpedance rise occurs and the catheter must be removed from the body andthe tip electrode cleaned.

In a typical application of RF current to the endocardium, circulatingblood provides some cooling of the ablation electrode. However, there istypically a stagnant area between the electrode and tissue which issusceptible to the formation of dehydrated proteins and coagulum. Aspower and/or ablation time increases, the likelihood of an impedancerise also increases. As a result of this process, there has been anatural upper bound on the amount of energy which can be delivered tocardiac tissue and therefore the size of RF lesions. Historically, RFlesions have been hemispherical in shape with maximum lesion dimensionsof approximately 6 mm in diameter and 3 to 5 mm in depth.

It is desirable to reduce or eliminate impedance rises and, for certaincardiac arrhythmias, to create larger lesions. One method foraccomplishing this is to irrigate the ablation electrode, e.g., withphysiologic saline at room temperature, to actively cool the ablationelectrode instead of relying on the more passive physiological coolingof the blood. Because the strength of the RF current is no longerlimited by the interface temperature, current can be increased. Thisresults in lesions which tend to be larger and more spherical, usuallymeasuring about 10 to 12 mm.

The effectiveness of irrigating the ablation electrode is dependent uponthe distribution of flow within the electrode structure and the rate ofirrigation flow through the tip. Effectiveness is achieved by reducingthe overall electrode temperature and eliminating hot spots in theablation electrode which can initiate coagulum formation.

More channels and higher flows are more effective in reducing overalltemperature and temperature variations, i.e., hot spots. However, thecoolant flow rate should be balanced against the amount of fluid thatcan be injected into a patient and the increased clinical load requiredto monitor and possibly refill the injection devices during a procedure.In addition to irrigation flow during ablation, a maintenance flow,typically at a lower flow rate, is required throughout the procedure toprevent backflow of blood flow into the coolant passages. Thus reducingcoolant flow by utilizing it as efficiently as possible is a desirabledesign objective.

The arrangement of conventional internal catheter components such asirrigation lumens, location sensor and related electrical leads islimited by available cross-sectional area of the tip electrode. Thelimiting direction is typically in the radial direction emanating fromthe axial centerline of the tip electrode radiating to the outerperiphery. Conventional irrigation tubings or the through-passage formedin the tip electrode receiving an irrigation tubing has a circularcross-section and is therefore limited in size by this radial dimension.Furthermore it is generally desirable to have the largest possible fluidlumen in order to minimize hydraulic resistance/pressure drop over thelength of the catheter shaft. These factors can often result in a designusing either a smaller-than-desired fluid lumen, or a two-piece tubingpossessing a larger diameter in the catheter shaft and a smallerdiameter coupler at the tip electrode. The inclusion of the couplerresults in an additional adhesive bond joint which contributes to ahigher risk of fluid leaks.

Moreover, conventional irrigated ablation tip electrodes are designed assolid monolithic structures with internal fluid paths and fluid portswhere the internal fluid paths are much longer, if not two, three, orfour times longer, than the size of the fluid port. Where fluid flowalong the length of the catheter shaft is assumed to be laminar,Poiseuille's law states that pressure drop over a distance isproportional to the flow rate multiplied by the hydraulic resistance,where hydraulic resistant relates fluid viscosity and conduit geometry.Because of the temperature of the irrigating fluid and consequently thehigh viscosity of the fluid relative to the port diameter, and thelength of the irrigation tubing, a significant amount of energy isrequired to pump the fluid to the tip electrode.

Conventional irrigated ablation tip electrodes also typically have amuch greater total fluid output area compared to fluid input area wherethe fluid output area is a two, three or four multiple of the fluidinput area. As such, the flow of irrigation fluid out of the outletfluid ports is primarily governed by the inertia of the fluid. Applyingthe law of conservation where the flow of the fluid into the electrodeequals the flow of fluid out of the electrode, a significant amount ofenergy is used not only to pump the fluid to the tip electrode, but toprovide the fluid with a desirable exit velocity from the electrode.

Another concern with conventional irrigated ablation tip electrodes isthe axially variability of fluid mass flow rate through the tipelectrode. Fluid entering a proximal end of a tip electrode chambercarries momentum in the axial direction such that more fluid tends toexit the fluid ports at the distal end compared to fluid ports on theradial side of the tip electrode. Such uneven distribution of fluid cancause undesirable “hot spots” which can compromise the size and qualityof the lesions and require interruption of the ablation procedure sothat coagulation can be removed from the tip electrode.

Ablation electrodes using a porous material structure can provideefficient coolant flow. The porous material in which tiny particles aresintered together to form a metallic structure provides a multiplicityof interconnected passages which allow for efficient cooling of anelectrode structure. However, because the particles are sinteredtogether, there can be concerns with particles detaching from theelectrode and entering the bloodstream.

Irrigation tip ablation electrodes employing thin shells are known,where the shells have a plurality of irrigation fluid ports. The fluidports are typically formed using sinker electrical discharge machining(EDM) technology. Although the sinker EDM process creates precise,minute geometries, it is typically an extremely slow process, with asingle irrigation port taking upwards of five minutes to completelyform.

Accordingly, it is desirable that a catheter be adapted for mapping andablation with improved irrigation fluid flow by means of more efficientuse of the space in the tip electrode that avoids the introduction ofadditional bonding joints. It is desirable that an irrigated tipelectrode use provides an internal fluid path that has a betterconsideration and utilization of inherent fluid dynamics for improvedfluid flow and cooling of the tip electrode. Moreover, it is desirablethat irrigation ports be formed utilizing a more time and cost efficientprocess which would improve manufacturing capacity and also reduce unitcost.

SUMMARY OF THE INVENTION

The present invention is directed to a catheter adapted for mapping andablating heart tissue with improved irrigation fluid flow into and outof the tip electrode. By considering and applying fluid characteristicsand dynamics, the ablation tip electrode efficiently uses space anddistributes fluid more uniformly and with higher velocity withoutnecessarily using more power and energy at the irrigation fluid pumpsource or increasing fluid load on the patient.

In one embodiment, an irrigated ablation catheter includes an elongatedcatheter body, a deflectable section distal to the catheter body and anablation tip electrode. The tip electrode has a two piece designcomprising a thin outer shell defining a cavity, and an internal memberthat fits inside the shell. The shell has a predetermined plurality offluid ports, each with a predetermined diameter and each contributing toa total fluid output area of the tip electrode. The internal member hasa plug member and a baffle member. The plug member includes a fluidinlet into the cavity of the tip electrode where the fluid inlet has apredetermined cross-sectional shape defining a fluid input area.Moreover, the cavity is designed to function as a plenum chamber byproviding a variable inner cross-section so that momentum of the fluidentering the chamber is diffused and axial variability of fluid massflow rate through the tip electrode fluid ports is reduced.

In a more detailed embodiment, the catheter of the present invention hasa tip electrode wherein the diffusion ratio of total fluid output areato fluid input area that is less than 2.0, and a fluid port ratio of tipelectrode shell thickness to fluid port diameter that is less than 3.25.Moreover, the tip electrode also has a fluid inlet aspect ratio greaterthan 1.0 where the fluid inlet has a noncircular (for example, oval orelliptical) radial cross-section defined by a wider dimension along oneaxis and a narrower dimension along another axis. The plenum chamber hasan inner flow contour, for example, a bottleneck, where a narrowproximal portion opens to a wider distal portion so that fluid pressureincreases while axial fluid velocity decreases which decreases axialmomentum for a more uniform distribution of fluid in the tip electrodeand thus more uniform flow of fluid exiting the fluid port.

In a detailed embodiment, the internal member includes a distal bafflemember and a proximal plug member connected by a stem. Distal ends ofirrigation tubing, electrode lead wires, puller wires and thermocouplewires are anchored in the plug member. The plug has an inlet passageallowing the irrigation tubing to deliver fluid into the tip electrode.The inlet passage is off-axis and has a noncircular cross-sectionalshape which efficiently uses the limited space in the tip electrode. Thebaffle member is shaped to diffuse fluid entering the tip electrode fromthe irrigation tubing as the fluid flows through the bottleneck of theplenum chamber. The baffle member is positioned on axis as it houses anelectromagnetic position sensor advantageously in a centered distalposition in the tip electrode. A cable for the sensor extends proximallyfrom the sensor through a passage extending through the baffle member,the stem and the plug member.

As another feature of the present invention, the fluid ports have atapered cylindrical configuration with divergent walls that are formedby laser drilling. Laser drilling offers advantages, including noconsumable/degradable tools, when compared to traditional screw machineor sinker EDM processes. The absence of degradable tooling allows laserdrilling to be a more efficient process, because in-process adjustmentis not required to compensate for tool wear. Additionally, the lasercutting mechanism is orders of magnitude faster than a comparable EDMprocess, with a single fluid port being drilled in seconds.

The divergent walls of laser drilled fluid ports are a result oftransverse modes present in the focused laser beam and its interactionwith surrounding substrate material (namely, the shell). The degree oftaper is relatively small, ranging between 0 and 6 degrees, but thetaper advantageously provides an increase in volumetric flow rate and adecrease in hydraulic resistance.

In one embodiment, each fluid port has a tapered configuration, forexample, a frustoconical configuration defined by a taper angle, with asmaller inlet diameter and a larger outlet diameter, where the smallerinlet diameter ranges between about 0.003 inch and 0.005 inch. The taperangle may range between about 0 degrees to 6 degrees. Thickness of theelectrode shell may range between about 0.003 inch to 0.004 inch.

BRIEF DESCRIPTION OF THE DRAWINGS

The patent or application file contains at least one drawing executed incolor. Copies of this patent or patent application publication withcolor drawings will be provided by the Office upon request and paymentof the necessary fee.

These and other features and advantages of the present invention will bebetter understood by reference to the following detailed descriptionwhen considered in conjunction with the accompanying drawings wherein:

FIG. 1 is a side view of an embodiment of a catheter of the presentinvention.

FIG. 2A is a side cross-sectional view of the catheter of FIG. 1,including a junction between a catheter body and a deflectableintermediate section, taken along a diameter.

FIG. 2B is a side cross-sectional view of the catheter of FIG. 1,including a junction between a catheter body and a deflectableintermediate section, taken along a diameter generally orthogonal to thediameter of FIG. 2A.

FIG. 2C is an end cross-sectional view of the intermediate section ofFIGS. 2A and 2B, taken along line 2C-2C.

FIG. 3 is a perspective view of a distal section of the catheter of FIG.1.

FIG. 3A is a side cross-sectional view of the distal section of FIG. 3,taken along a first diameter.

FIG. 3B is a side cross-sectional view of the distal section of FIG. 3,taken along a second diameter generally orthogonal to the firstdiameter.

FIG. 4 is a perspective view of the distal section of FIG. 3, withselected components removed for better viewing of the interior of thedistal section, including an embodiment of an internal member.

FIG. 5 is a perspective view of a proximal end of the internal member ofFIG. 4.

FIG. 6 is a distal end view of the internal member of FIG. 5.

FIG. 7 illustrate various noncircular shapes.

FIG. 8 is a perspective view of an alternate embodiment of a tipelectrode of the present invention.

FIG. 9 is a perspective view of another alternate embodiment of a tipelectrode of the present invention.

FIG. 10 is a side cross-sectional view of a fluid port with a rightcircular cylindrical configuration with straight and parallel walls.

FIG. 11 is a side cross-sectional view of a fluid port with a taperedcylindrical configuration with divergent walls.

FIG. 12 is a Table of Standard Discharge Coefficients.

FIG. 13 is a graph showing Discharge Coefficient effect on Pressureversus Volumetric Flowrate Sensitivity.

FIG. 14 is a graph showing Computational Fluid Dynamic of IrrigationPort Pressure Drop Sensitivity at 8 ml/min.

FIG. 15 is a graph showing Computational Fluid Dynamic of IrrigationPort Pressure Drop at 15 ml/min.

FIG. 16 is a Regression Table for Irrigation Port Pressure Drop Model

FIG. 17 is a schematic representation of irrigated tip shell hydraulicas an electrical circuit.

FIG. 18 is a schematic representation of the irrigated tip shell with 56fluid ports as a parallel resistant network analog.

FIG. 19 is a diagram of a flow fixture for characterizing hydraulicresistance.

FIG. 20 is a chart showing summary of results of various portconfigurations characterized by the flow fixture of FIG. 19.

FIG. 21 is a graph showing Pressure versus Bulk Volumetric Flowrate

FIG. 22 is a chart showing Bulk Hydraulic Resistance for various portconfigurations.

FIG. 23 is a graph correlating laser drilled port geometry to EDM portgeometry.

FIG. 24 is the graph of FIG. 23 with normalized flow rate.

FIG. 25 is a chart showing ranges of hydraulic resistance of a singleEDM port.

FIG. 26 is a graph showing diameter-based interpolation of EDM portpressure.

FIG. 27 is a graph showing hydraulic resistance performance envelope forEDM and laser drilled ports.

FIG. 28 is a chart showing hydraulic resistance of laser-drilled portsrelative to validated specification limits.

FIG. 29 is a graph showing the sensitivity of hydraulic resistancerelative to a fluid port (“orifice”) diameter of 0.005 inch.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates an embodiment of a catheter 10 with improvedirrigation flow through a tip ablation electrode 17. The tip electrodeis configured to promote fluid flow into the tip electrode anddispersion of fluid therein in providing more uniform fluid coverage andflow at all locations on the exterior of the tip electrode. The catheteris therefore operable at lower flow rates with lower fluid load on thepatient while providing improved cooling of the tip electrode. Moreover,a high fluid exit velocity at the tip electrode provides a “jetting”action that aids in creating a fluid boundary layer around the tipelectrode which reduces the occurrence rate of char and/or thrombusduring ablation. Fluid, e.g., saline or heparinized saline, can betransported to the ablation site from the tip electrode to cool tissue,reduce coagulation and/or facilitate the formation of deeper lesions. Itis understood that other fluids can be delivered, as well, including anydiagnostic and therapeutic fluids, such as neuroinhibitors andneuroexcitors.

The catheter 10 has an elongated catheter body 12 with proximal anddistal ends, an intermediate deflectable section 14 at the distal end ofthe catheter body 12, and a distal section 15 with the irrigated mappingand ablation tip electrode 17. The catheter also includes a controlhandle 16 at the proximal end of the catheter body 12 for controllingdeflection (single or bi-directional) of the intermediate section 14.

With reference to FIGS. 2A and 2B, the catheter body 12 comprises anelongated tubular construction having a single, axial or central lumen18. The catheter body 12 is flexible, i.e., bendable, but substantiallynon-compressible along its length. The catheter body 12 can be of anysuitable construction and made of any suitable material. A presentlypreferred construction comprises an outer wall 20 made of polyurethaneor PEBAX. The outer wall 20 comprises an imbedded braided mesh ofstainless steel or the like to increase torsional stiffness of thecatheter body 12 so that, when the control handle 16 is rotated, theintermediate section 14 of the catheter 10 will rotate in acorresponding manner.

The outer diameter of the catheter body 12 is not critical, but ispreferably no more than about 8 french, more preferably 7 french.Likewise the thickness of the outer wall 20 is not critical, but is thinenough so that the central lumen 18 can accommodate puller members(e.g., puller wires), lead wires, and any other desired wires, cables ortubings. If desired, the inner surface of the outer wall 20 is linedwith a stiffening tube 22 to provide improved torsional stability. Adisclosed embodiment, the catheter has an outer wall 20 with an outerdiameter of from about 0.090 inch to about 0.94 inch and an innerdiameter of from about 0.061 inch to about 0.065 inch.

Distal ends of the stiffening tube 22 and the outer wall 20 are fixedlyattached near the distal end of the catheter body 12 by forming a gluejoint 23 with polyurethane glue or the like. A second glue joint 25 isformed between proximal ends of the stiffening tube 20 and outer wall 22using a slower drying but stronger glue, e.g., polyurethane.

Components that extend between the control handle 16 and the deflectablesection 14 pass through the central lumen 18 of the catheter body 12.These components include lead wires 30 for the tip electrode 17 and ringelectrodes 22 on the distal section 15, an irrigation tubing 38 fordelivering fluid to the distal section 15, a cable 33 for a positionlocation sensor 34 carried in the distal section, puller wire(s) 32 fordeflecting the intermediate section 14, and a pair of thermocouple wires41, 42 to sense temperature at the distal tip section 15.

Illustrated in FIGS. 2A, 2B and 2C is an embodiment of the intermediatesection 14 which comprises a short section of tubing 19. The tubing alsohas a braided mesh construction but with multiple off-axis lumens, forexample lumens 26, 27, 28 and 29. The first lumen 26 carries a pullerwire 32 for deflection of the intermediate section. For bi-directionaldeflection, the diametrically opposing second lumen 27 can carry asecond puller wire 32. The third lumen 28 carries the lead wires 30, thethermocouple wires 41 and 42, and the sensor cable 33. The fourth lumen29 carries the irrigation tubing 38.

The tubing 19 of the intermediate section 14 is made of a suitablenon-toxic material that is more flexible than the catheter body 12. Asuitable material for the tubing 19 is braided polyurethane, i.e.,polyurethane with an embedded mesh of braided stainless steel or thelike. The size of each lumen is not critical, but is sufficient to housethe respective components extending therethrough.

A means for attaching the catheter body 12 to the intermediate section14 is illustrated in FIGS. 2A and 2B. The proximal end of theintermediate section 14 comprises an outer circumferential notch 23 thatreceives an inner surface of the outer wall 20 of the catheter body 12.The intermediate section 14 and catheter body 12 are attached by glue orthe like.

If desired, a spacer (not shown) can be located within the catheter bodybetween the distal end of the stiffening tube (if provided) and theproximal end of the intermediate section. The spacer provides atransition in flexibility at the junction of the catheter body andintermediate section, which allows this junction to bend smoothlywithout folding or kinking. A catheter having such a spacer is describedin U.S. Pat. No. 5,964,757, the disclosure of which is incorporatedherein by reference.

Each puller wire 32 is preferably coated with Teflon®. The puller wirescan be made of any suitable metal, such as stainless steel or Nitinoland the Teflon coating imparts lubricity to the puller wire. The pullerwire preferably has a diameter ranging from about 0.006 to about 0.010inch.

As shown in FIG. 2B, portion of each puller wire 32 in the catheter body12 passes through a compression coil 35 in surrounding relation to itspuller wire. The compression coil 35 extends from the proximal end ofthe catheter body 12 to the proximal end of the intermediate section 14.The compression coil is made of any suitable metal, preferably stainlesssteel, and is tightly wound on itself to provide flexibility, i.e.,bending, but to resist compression. The inner diameter of thecompression coil is preferably slightly larger than the diameter of thepuller wire. Within the catheter body 12, the outer surface of thecompression coil 35 is also covered by a flexible, non-conductive sheath39, e.g., made of polyimide tubing.

Proximal ends of the puller wires 32 are anchored in the control handle16. Distal ends of the puller wires 32 are anchored in the distalsection 15 as described further below. Separate and independentlongitudinal movement of the puller wire 32 relative to the catheterbody 12, which results in, respectively, deflection of the intermediatesection 14 and distal section 15 along a plane, is accomplished bysuitable manipulation of a deflection member of the control handle 16.Suitable deflection members and/or deflection assemblies are describedin co-pending U.S. application Ser. No. 12/346,834, filed Dec. 30, 2008,entitled DEFLECTABLE SHEATH INTRODUCER, and U.S. application Ser. No.12/127,704, filed May 27, 2008, entitled STEERING MECHANISM FORBI-DIRECTIONAL CATHETER, the entire disclosures of both of which arehereby incorporated by reference.

At the distal end of the intermediate section 14 is the distal tipsection 15 that includes the tip electrode 17 and a relatively shortpiece of connection tubing or covering 24 between the tip electrode 17and the intermediate section 14. In the illustrated embodiment of FIGS.3 and 4, the connection tubing 24 has a single lumen which allowspassage of the tip and ring electrodes lead wire 30, the sensor cable33, thermocouple wires 41 and 42, the puller wires 32, and theirrigation tubing 38 into the tip electrode 17. The single lumen of theconnection tubing 24 allows these components to reorient themselves asneeded from their respective lumens in the intermediate section 14toward their location within the tip electrode 17. In the disclosedembodiment, the tubing 24 is a protective tubing, e.g., PEEK tubing,having a length ranging between 6 mm and 12 mm, more preferably about 11mm. It is noted that selected components, including the tip and ringelectrode lead wires 30 are not shown for better clarity of othercomponents and structure of the tip electrode.

Better seen in FIGS. 3A and 3B, the tip electrode 17 defines alongitudinal axis and is of a two piece configuration that includes anelectrically conductive shell 50, an internal member 52 and a cavity orchamber 51 generally surrounded and enclosed by the shell and internalmember. The shell is elongated, with a tubular or cylindrical shape. Theshell has a closed and rounded atraumatic distal end 53 and an openproximal end 54 that is sealed by the internal member. In theillustrated embodiment, the shell is radially symmetrical where theradial cross section of the shell 50 is circular, but it is understoodthat the radial cross section may be any shape as desired. The shell hasa distal portion 50D, a proximal portion 50P and a short tapered portion50T therebetween connecting the two portions. The cavity 51 extends thelength of the shell such that there is an inner dimension or radius RDin the distal portion 50D, an inner dimension or radius RT in thetapered portion 50T and an inner dimension or radius RP in the proximalportion 50P where the radii have the following relationships: RD>RP andRD>RT>RP. In the disclosed embodiment, RD is about 1.15 mm, RP is about1.0 mm and RT is about 1.075 mm. A length of the shell from the distalend 53 to the proximal end 54 ranges between about 2 mm to 12 mm, andpreferably between about to 3 mm to 10 mm, and more preferably about 7.5mm.

The internal member 52 inside the proximal portion of the shell has alength that is about half of the length of the shell. The internalmember is radially symmetrical and has a distal portion (or bafflemember) 58 and a proximal portion (or plug member) 59 that are connectedby a narrow on-axis stem 60. The baffle member has a greater length andthe plug member has a lesser length. In the disclosed embodiment,internal member 52 is radially symmetrical and its length is about 3.0mm to 4.0 mm with the length of the baffle member 58 being about twicethe length of the plug member 59.

With reference to FIGS. 5 and 6, the plug member 59 has a circular crosssection that corresponds with the circular cross section of the proximalportion 50P of the shell 50 so that it forms a snug fit in providing afluid-tight seal at the proximal end 54 of the tip electrode 17. Theplug member 59 seals the interior cavity 51 of the shell 50, and theshell and the plug member facilitate the provision of a plenum conditionwithin the cavity; that is, where fluid is forced or delivered into itfor a more uniform distribution through fluid ports 44 formed in theshell, as discussed further below.

The baffle member 58 has a radial cross-section that is nonconforming tothe inner radial cross section of the shell surrounding the bafflemember, so that separate gaps or pathways are provided for fluid flowingthrough the tip electrode. In the disclosed embodiment, baffle member 58has a polygonal cross-section, for example, a triangular cross-sectionas illustrated, with a plurality of angled baffles or generally flatsurfaces 62. Truncated corners 63 between the surfaces are dimensionedfor contact with inner surface of the shell wall. The internal member 52has an on-axis passage 64 extending through the entirety of its length,including the baffle member 58, the stem 60 and the plug member 59. Adistal portion 64D of the passage extending through the baffle member 58houses a proximal portion of the position sensor 34. A proximal (andnarrower) portion 64P of passage 64 extending through the stem 60 andthe plug member 59 allows the sensor cable 33 to extend proximally fromthe sensor. A junction between the distal and proximal portion of thepassage acts as a stop 64J abutting against the proximal end of theposition sensor 34. In the disclosed embodiment, the length of thedistal portion 64D of the passage is about half of the length of theposition sensor 34. A distal portion of the sensor 34 is sealed andprotected from surrounding fluid by a nonconducting, biocompatibletubing 66, e.g., polyimide tubing, whose distal end extends slightlybeyond the distal end of the position sensor 34 and is sealed by a plugof sealant material 67. The distal end of the tubing 66 is proximal ofthe distal end 53 of the shell 50 so there is a space or gap 65 forfluid to circulate and reach the distal end of the shell.

The stem 60 of the internal member 52 has a generally circular radialcross-sectional shape, with a diameter slightly greater than thediameter of the passage 64P. Its small diameter allows fluid exiting theirrigation tubing 38 to impinge on the proximal surface of the bafflemember 58, circulate and better fill the chamber 51 of the tip electrodebefore flowing distally.

On a proximal end of the plug member 59, a circumferential lip 70 isformed. With the tip electrode 17 assembled, the proximal end 54 of theshell 50 abuts a distal surface of the lip. The lip prevents the shell50 from being installed improperly over the internal member 52. Inparticular, the lip ensures the gap 65 between the distal ends of thebaffle member and the shell, while the truncated corners of the bafflemember ensure axial alignment between the shell and the internal member.A distal portion of the connection tubing 24 extends over the lip 70 andthe proximal portion 50P of the shell 50 such that a distal end of thetubing 24 is at or near the tapered portion 50T of the shell.

On a proximal surface of the plug member 59, blind holes 71, 73 and 74are provided. A distal end of each puller wire 32 is anchored in holes71 by means of a ferrule 31 as known in the art. Distal end of tipelectrode lead wire 30 is anchored in hole 74, and distal ends ofthermocouple wires 41, 42 are anchored in hole 73. As mentioned, theon-axis through-passage 64 houses the sensor 34 and the cable 33.Another through-passage, for example, an off-axis through-passage 75, isprovided in the plug member 59 to receive a distal end of the irrigationtubing 38 which feeds fluid into the enclosed chamber 51 of the tipelectrode 17. In accordance with a feature of the present invention, thethrough-passage 75 has a predetermined cross-sectional shape thatefficiently uses the limited space on the proximal surface of the plugmember 59. That is, the tip electrode 17 including the internal member52 considers a fluid inlet aspect ratio Ratio_(INLET), as defined byEquation (1) below:Ratio_(INLET) =L/W  Eqn (1)

-   -   where:        -   L is a greater (or length) dimension;        -   W is a lesser (or width) dimension; and            In particular, the plug member 59 has an irrigation inlet            passage radial cross-section wherein the ratio Ratio_(INLET)            is limited to being greater than or equal to 1.0, per            Equation (2), and preferably not greater than 10 as per            Equation (2a) as follows:            Ratio_(INLET)≧1  Eqn (2)

In the illustrated embodiment, the oval or elliptical cross-sectionalshape of the fluid inlet passage 75 is defined by Equations (1) and (2),including but not limited to where the dimensions are generallyorthogonal to each other. Although the illustrated embodiment is an ovalor ellipse, it is understood that the present invention is directed toan irrigation inlet with any noncircular shapes, including irregularcircles, regular or irregular polygons, and “ameobic” shapes, forexample, kidney-bean, crescent, peanut, hourglass, and pear shapes, asshown in FIG. 7. The noncircular cross-sectional shapes that can beassumed by the passage can also be formed by combinations of a pluralityof two or more irrigation tubings 38 in contact and/or close proximity.Indeed, a bundle of irrigation tubings can be inserted into the inletpassage 75 so long as the passage is effectively sealed at its proximalend, for example, by means of a funnel seal or sleeve. In fact, a largenumber of different noncircular shapes is subject only to the layout andarrangement of the other components in the tip electrode, means ofmanufacturing the plug member in forming the inlet passage and/or meansof sealing the irrigating tubing(s) to the inlet passage. The presentinvention recognizes that a noncircular cross-section shape uses spacewithin the tip electrode more efficiently than a circular shape.Irrigation tubing(s) constructed of flexible material, e.g., polyimide,can readily adapt to the shape of the through-passage allowing thetubing(s) to be continuous without the need for bond joints along theirlength. As illustrated in FIG. 3B, a continuous irrigation tubing 38 isused, at least through the distal section 15. Its flexibility andelasticity allow different cross sections along its length. A distalportion 38D of the tubing extending generally within the connectiontubing 24 has a cross section and size similar to that of the irrigationthrough-passage 75. A portion 38P proximal the connection tubing 24 hasa more conventional circular cross-section.

The shell 50 is constructed of a biocompatible metal, including abiocompatible metal alloy. A suitable biocompatible metal alloy includesan alloy selected from stainless steel alloys, noble metal alloys and/orcombinations thereof. In one embodiment, the shell is constructed of analloy comprising about 80% palladium and about 20% platinum by weight.In an alternate embodiment, the shell is constructed of an alloycomprising about 90% platinum and about 10% iridium by weight. The shellcan formed by deep-drawing manufacturing process which produces asufficiently thin but sturdy shell wall 50W that is suitable forhandling, transport through the patient's body, and tissue contactduring mapping and ablation procedures. In a disclosed embodiment, theshell wall 50W has a generally uniform thickness T ranging between about0.003 in and 0.010 in, preferably between about 0.003 in and 0.004 in,and more preferably about 0.0035 in. While the deep drawn method is wellsuited to manufacturing the shell with a sufficiently thin wall, it isunderstood that other methods, such as drilling and/or casting/molding,can also be used.

With the shell wall sufficiently thin, an electrical discharge machining(EDM) process can be employed to form a plurality of fluid ports ororifices 44 in the shell wall 50W of the distal portion 50D that allowfluid communication between the chamber 51 and outside the shell. In adisclosed embodiment, the plurality of ports 44 ranges between about 20and 96, preferably between about 30 and 60, more preferably about 56. Adiameter D of each fluid port ranges between about 0.003 in, and 0.007in., preferably between about 0.003 inch and 0.004 inch, and morepreferably about 0.0035 inch.

In the disclosed embodiment, there are 56 ports, arranged in sixcircumferential rows, where five rows R1-R5 have 10 ports each, and adistal row R6 has six ports. The ports of rows R1-R5 are generallyequidistant from each other, although the ports of adjacent rows areoffset from each other such that each port is equidistant to four or sixadjacent ports. A most distal ten-port row R5 is located at the roundeddistal portion of the shell. The row (or circle) R6 is on a flat ornearly flat distal end 53 of the shell. The six ports of the row R6 areequi-angular on the circle.

In accordance with another feature of the present invention, the tipelectrode 17 including the shell 50 has a configuration that considers afluid port ratio Ratio_(PORT) as defined by Equation (3) below:Ratio_(PORT) =T/D  Eqn (3)

-   -   where:        -   T=thickness of shell wall; and        -   D=diameter of a fluid port

In particular, the tip electrode of the present invention has the fluidport aspect ratio Ratio_(PORT) being less than 3.25 as per Equation (4)below, preferably less than or equal to about 1.5 as per Equation (5),and more preferably less than or equal to about 1.0, as per Equation (6)below:Ratio_(PORT)<3.25  Eqn. (4)Ratio_(PORT)≦1.5  Eqn. (5)Ratio_(PORT)≦1.0  Eqn. (6)

Such a thin shell configuration with fluid ports 44 of a predetermineddiameter D, including where the shell wall thickness T is less than thefluid port diameter D, fosters a fluid flow through the tip electrodethat can be characterized as thin plate orifice flow which operates by adistinct set of characteristics, as discussed below.

Equation (7) below is an expression of Bernoulli's law based on theprinciple of conservation of energy (pressure and kinetic energy onlywhen applying the assumption of a common flow height such that potentialenergy can be ignored):

$\begin{matrix}{{\frac{P_{OUT}}{\rho} + \frac{V_{OUT}^{2}}{2}} = {\frac{P_{IN}}{\rho} + \frac{V_{IN}^{2}}{2} + \frac{\Delta\; P_{{OUT}\text{-}{IN}}}{\rho}}} & {{Eqn}.\mspace{14mu}(7)}\end{matrix}$

-   -   Where:        -   P_(OUT)=discharge ambient pressure outside tip electrode        -   P_(IN)=upstream pressure at distal end of irrigation tubing            inside tip electrode        -   ΔP_(OUT-IN)=pressure loss in fluid port        -   V_(OUT)=velocity outside the tip electrode        -   V_(IN)=velocity inside the tip electrode        -   ρ=density

Applying the assumption that pressure loss in the fluid port is low tonegligible (pressure drop is included with coefficient of discharge),and expressing velocities V_(OUT) and V_(IN) in terms of flow rate anddiameter, per Equations (8) and (9) below:

$\begin{matrix}{V_{OUT} = \frac{4\; Q}{\pi\; D_{OUT}^{2}}} & {{Eqn}\mspace{14mu}(8)} \\{V_{IN} = \frac{4\; Q}{\pi\; D_{IN}^{2}}} & {{Eqn}\mspace{14mu}(9)}\end{matrix}$

-   -   where:        -   {dot over (Q)}=volumetric flow rate        -   D_(IN)=theoretical diameter leading into the fluid port,            estimated by separation distance between adjacent fluid            ports        -   D_(OUT)=diameter of fluid port            the pressure drop through the fluid can be expressed as            Equation (10) below:

$\begin{matrix}{\frac{P_{IN} - P_{OUT}}{\rho} = {{1/2}\left\{ {\left( {16\;{Q^{2}/\pi^{2}}D_{OUT}^{4}} \right) - \left( {16\;{Q^{2}/\pi^{2}}D_{IN}^{4}} \right)} \right\}}} & {{Eqn}\mspace{14mu}(10)}\end{matrix}$

Because the fluid port is small compared to the spacing between thefluid ports, where D_(IN) is much greater than D_(OUT), Equation (10)can be simplified to Equation (11) below, which shows that as thediameter of the fluid port increases, the hydraulic resistance decreasesby the fourth power.ΔP=ρ(8Q ²)/(π² D _(OUT) ⁴)  Eqn (11)

Another feature of the present invention is the tip electrode considersa diffusion Ratio_(DIF), as shown in Equation (12) below:Ratio_(DIF) =A _(OUTPUT) /A _(INPUT)  Eqn. (12)

-   -   where:        -   A_(OUTPUT) is the total area of all fluid ports of the shell        -   A_(INPUT) is the area of the irrigation tubing distal end            inlet

In particular, the tip electrode configuration of the present inventionlimits the diffusion Ratio_(DIF) to less than about 2.0 per Equation(13a), preferably less than about 1.8 per Equation (13b), and morepreferably less than about 1.3 per Equation (13c) below:2.0>Ratio_(DIFFUSION)  Eqn (13a)1.8>Ratio_(DIFFUSION)  Eqn (13b)1.3>Ratio_(DIFFUSION)  Eqn. (13c)

Bernoulli's law of Equation (7) above assumes that the fluid isincompressible and suffers no friction as it moves through a pipe. Inreality, velocity varies throughout the fluid depending on the viscosityof the fluid. For sufficiently small velocities, such as those throughirrigated catheters, the flow is generally laminar, i.e. layered. Withlaminar flow, velocities vary parabolically across a pipe with acircular cylindrical cross section. As the velocity increases past acritical value, depending upon the viscosity and density of the fluid,eddies appear and the flow becomes turbulent.

The laminar flow through a pipe is described by the Hagen-Poiseuillelaw, per Equation (14) below which states that volume of fluid flowingper unit time is proportional to the pressure difference ΔP between theends of the pipe and the fourth power of its radius r:

$\begin{matrix}{Q = \frac{\pi\;\Delta\;\Pr^{4}}{8\;\eta\; L}} & {{Eqn}.\mspace{14mu}(14)}\end{matrix}$

-   -   Where:    -   Q=volume of fluid flowing per unit time    -   ΔP=pressure difference between the ends of the pipe    -   r=radius of the pipe    -   L=length of the pipe    -   η=dynamic viscosity, a characteristic of a given fluid

By solving for ΔP, Equation (14) can be expressed with the change inpressure as a function of flow rate and radius, as per Equation (15)below:

$\begin{matrix}{{\Delta\; P} = \frac{8\; Q\;\eta\; L}{\pi\; r^{4}}} & {{Eqn}.\mspace{14mu}(15)}\end{matrix}$

Thus, an increase in the radius results in a significant decrease inpressure change, and vice versa. And, because hydraulic resistance R_(H)is a function of viscosity and the geometries of the pipe, as perEquation (16) below, an increase in radius results in a significantdecrease in hydraulic resistance, and vice versa:

$\begin{matrix}{R_{H} = \frac{8\;\eta\; L}{\pi\; r^{4}}} & {{Eqn}.\mspace{14mu}(16)}\end{matrix}$

In the present invention, the shell of the tip electrode advantageouslycapitalizes on the inverse dependency between change in pressure andfluid port radius, and between hydraulic resistance and fluid portradius by utilizing a thin tip electrode shell wall 50W with apredetermined plurality of fluid ports 44. Because of the relativelysmall thickness T of the shell wall (taken to be the “length L” in Eqn(16)), the fluid ports can be readily manufactured in a variety of sizesand radius (taken to be the “radius r” in Eqn (16)) such that the fluidport ratio is less than 3.25 per Eqn (4) above, preferably less thanabout 1.5 per Eqn (5), and more preferably less than about 1.0 per Eqn(6). As the fluid port ratio approaches or becomes less than 1.0, thefluid flow through the ports can be characterized as “thin plate orificeflow.” Moreover, with a predetermined plurality of fluid ports of apredetermined radius or diameter, the diffusion ratio of a total outputarea (e.g., number of ports in tip electrode shell multiplied by area ofeach port) to input area (e.g., cross-sectional area of inlet 75) can bereadily determined and limited to being less than 2.0 per Eqn (13a),preferably less than 1.8 per Eqn (13b), and more preferably less thanabout 1.3 per Eqn (13c). By reducing the diffusion ratio, the flow ofirrigation fluid is largely governed by back pressure of the fluidwithin the tip electrode. And, because total mass flow rate of the fluidin and out the tip electrode must conserved per Equation (7) above, areduced total output area is advantageously compensated for by higherfluid velocities at the fluid ports in creating “jetting action” at thetip electrode.

In accordance with yet another feature of the present invention, the tipelectrode 17, and in particular, the shell 50 and the chamber 51, have avariable internal cross section with a larger distal inner radialdimension or cross section in the distal portion 50D and a smallerproximal inner radial dimension or cross section in the proximal portion50P, with the tapered section 50T facilitating the transition of thechanging inner radial dimension therebetween. The tapered section may beat or near a midpoint along a length of the shell as illustrated but itcan also be closer to either the distal end or the proximal end. Whilean outer radial dimension of the shell along its length may be variableor not, it is the variable inner radial dimension along the length ofthe electrode that advantageously affects fluid flow and createsdesirable turbulence within the chamber to provide a plenum condition.

In keeping with Eqn (7), the expansion or increase in chamber volumefrom the bottleneck formation of the proximal portion 50P widening tothe distal portion 50D increases pressure and decreases velocity in thefluid flowing distally in the tip electrode. A plenum chamber effect iscreated which diffuses the momentum of the fluid, especially the axialcomponent of the momentum. As the momentum or the irrigating fluid isdiffused, axial variability of fluid mass flow rate through the tipelectrode fluid ports 44 is reduced. The overall effect of thisphenomenon is a more uniform irrigation fluid coverage and flowthroughout the chamber of the tip electrode and thus at all locations onthe exterior of the tip electrode via the ports 44.

As understood by one of ordinary skill in the art, the tip electrodeprovides an internal geometry that controls irrigation fluid flow axialvariation. However, the present invention includes an alternateembodiment wherein the density of fluid ports 44 (including theplurality of ports per unit area of the shell wall or surface) down thelength of tip electrode 17′ is varied, as shown in FIG. 8. Additionally,another alternate embodiment as shown in FIG. 9 provides a shell whereinthe diameter of the ports varies axially along the length of a tipelectrode 50″, including decreasing diameters toward the distal end. Ineither case, the effective fluid output area varies with the length ofthe tip electrode and compensates for the pressure drop in order toyield more uniform mass flow rates.

The ring electrodes 21 which are mounted on the connection tubing 24 canbe made of any suitable solid conductive material, such as platinum orgold, preferably a combination of platinum and iridium. The ringelectrodes can be mounted onto the connection tubing 24 with glue or thelike. Alternatively, the ring electrodes can be formed by coating thetubing 24 with an electrically conducting material, like platinum, goldand/or iridium. The coating can be applied using sputtering, ion beamdeposition or an equivalent technique. The number of the ring electrodeson the tubing 24 can vary as desired. The rings may be monopolar orbi-polar. In the illustrated embodiment, there is a distal monopolarring electrode and a proximal pair of bi-polar ring electrodes. Eachring electrode is connected to a respective lead wire 30R.

Each lead wire 30R is attached to its corresponding ring electrode byany suitable method. A preferred method for attaching a lead wire to aring electrode involves first making a small hole through the wall ofthe tubing 24. Such a hole can be created, for example, by inserting aneedle through the non-conductive covering and heating the needlesufficiently to form a permanent hole. The lead wire is then drawnthrough the hole by using a microhook or the like. The end of the leadwire is then stripped of any coating and welded to the underside of thering electrode, which is then slid into position over the hole and fixedin place with polyurethane glue or the like. Alternatively, each ringelectrode is formed by wrapping a lead wire 30R around thenon-conductive tubing 24 a number of times and stripping the lead wireof its own insulated coating on its outwardly facing surfaces.

The tip electrode 17 is electrically connected to a source of ablationenergy by the lead wire 30T. The ring electrodes 21 are electricallyconnected to an appropriate mapping or monitoring system by respectivelead wires 30R.

The lead wires 30T and 30R pass through the lumen 28 of the tubing 19 ofthe deflectable intermediate section 14 and the central lumen 18 of thecatheter body 12. The portion of the lead wires extending through thecentral lumen 18 of the catheter body 12, and proximal end of the lumen28 can be enclosed within a protective sheath (not shown), which can bemade of any suitable material, preferably polyimide. The protectivesheath is anchored at its distal end to the proximal end of theintermediate section 14 by gluing it in the lumen 28 with polyurethaneglue or the like. Each electrode lead wire has its proximal endterminating in a connector at the proximal end of the control handle 16.

The tip electrode of the present invention can operate at about 8ml/minute or lower for wattage below 30 and about 17 ml for wattagebetween 30 and 50. The reduction in fluid-loading on the patient in afive or six hour procedure can thus be very significant. Moreover, wherethe flow rate is regulated by a programmable pump, the flow rate caneven be lower for lower wattage.

With reference to FIG. 10, a fluid port 44 with a right circularcylindrical configuration is shown with diameter D ranging between about0.003 and 0.005 inch and shell thickness T ranging between about 0.003and 0.004 inch. The fluid port is manufactured with sinker EDMtechnology. A tungsten electrode is progressively plunged into shellwall to form individual irrigation ports by electrical erosion. With astraight, circular electrode, a right circular cylindrical irrigationport is formed with straight and parallel walls. This process isrepeated multiple times over the shell to form the desired plurality ofports.

With reference to FIG. 11, a fluid port 44′ with a tapered circularcylindrical or a frustoconical configuration, where an angle of taper aranges between about 0 to 10 degrees, and preferably between about 4 to6 degrees. In one embodiment, inner/inlet port diameter D1 rangesbetween about 0.003 inch and 0.004 inch, and outer/outlet port diameterD2 ranges between about 0.004 and 0.005 inch and shell thickness Tranges between about 0.003 inch and 0.004 inch. In accordance with afeature of the present invention, the taper angle α has a beneficialeffect on irrigation port flow, as described below.

With reference to FIG. 11, a study with the following parameters wasconducted which demonstrated the effect of taper angle on irrigationport flow:

-   -   Port Diameter D between 0.003 inch and 0.005 inch    -   Shell Thickness T between 0.003 inch and 0.004 inch    -   Bulk volumetric flow rate F between 8 ml/min and 15 ml/min    -   Taper Angle α of 0 to 6 degrees

Saline flow through a fluid port 44 can be theoretically modeled throughapplication of Bernoulli's equation. With the assumptions ofsteady-state incompressible flow and negligible frictional losses,Bernoulli's equation reduces to:

$\begin{matrix}{\overset{.}{Q} = {\frac{C_{d}}{\sqrt{1 - \left( \frac{d_{2}}{d_{1}} \right)^{4}}}A_{2}\sqrt{\frac{2\left( {\Delta\; P} \right)}{\rho}}}} & {{Eqn}\mspace{14mu}(17)}\end{matrix}$Where:

{dot over (Q)} is volumetric flow rate through the port

C_(d) is the discharge coefficient

A₂ is the area of the irrigation port

d₂ is the diameter of the irrigation port

d₁ is the upstream diameter (assumed to be port to port spacing onirrigated tip)

ΔP is the pressure drop across the irrigation port

ρ is the fluid density

Where the EDM and the laser drilling are centered with identicalinterior/inlet port diameters, the inputs to Bernoulli's equation (Eqn.17) are identical. The effect of the taper angle is therefore manifestedin different discharge coefficients C_(d). The discharge coefficient fora given port can be approximately determined by referencing variousfluid mechanics tables as shown in FIG. 12. Therein, it can be seen thattaper angle, inlet radiuses, and the ratio of wall thickness to portdiameter all effect C_(d)=1 being in perfect correlation withBernoulli's equation. As such, standardized tables should be used withcaution as their validity is heavily dependent on geometry and fluidconditions.

In general, if the assumption is made that all over variables with theexception of Cd are constant between straight and tapered nozzles,Bernoulli's equation can be reduced to:

$\begin{matrix}{{\Delta\; P} = \frac{{\overset{.}{Q}}^{2}}{C_{d}^{2}M}} & {{Eqn}\mspace{14mu}(18)}\end{matrix}$Where M is a proportionality constant based upon common port geometry.Plotting Eqn (18) yields a family of curves whose sensitivity isinversely proportional to C_(d) ², see FIG. 13. The plot illustratesthat it is not possible to fully characterize an irrigation port's flowperformance without empirical verification of the discharge coefficientC_(d).

An alternate approach to theoretical modeling is to use computationalnumerical methods. A computational fluid dynamic (CFD) analysis wasperformed on fluid flow through a single fluid port over varyingdiameters, taper angles, and volumetric flow rates. The results of themultiple CFD runs were loaded into Minitab in a response surface DOEmodel in order to efficiently view the experimental space as shown inFIGS. 14, 15 and 16. A central composite DOE design was utilized togenerate the CFD run combinations. The final response plots weregenerated using linear fits. In these plots, at both 8 ml/min and 15ml/min volumetric flow rate, the effect of taper angle is minimal evenat 12 degrees. Using the linear, multi-variable function generated inFIG. 16, the pressure drop for straight fluid port 44 (FIG. 10) andtapered fluid port 44′ (FIG. 11) can be estimated at less than 5%difference.ΔP=−0.000712(Taper Angle)−126.3(Port Diameter)+0.0104(Flow Rate)+0.456

Port Diameter Flow Rate Taper Pressure Drop % change [in] [ml/min] Angle[psi] from 0* 0.0035 15 0* 0.170 NA 6* 0.166 2.5% reduction 12*  0.1615.0% reduction

Furthermore, the Regression Table of FIG. 16 shows a P value of 0.938for Taper Angle, indicating that it has little to no statisticalsignificance and therefore has minimal effect on axial pressure droprelative to flow rate and inlet port diameter.

As discussed above, wall thickness of the tip electrode shell, andthereby the “length” of fluid port conduit affects the flow through theport. Because of the inability to precisely predict Cd, an alternateapproach would be to characterize the port's total hydraulic resistanceRH. Hydraulic resistance will effectively quantify the ration of forcerequired to move a unit volume of fluid through the port. In order tocharacterize RH for the fluid port, it is convenient to consider anelectric circuit analog. A simple resistor circuit can be constructed,which is analogous to the bulk fluid flow through the irrigated tipshell as shown in FIG. 17.

Using Ohm's Law, the electrical circuit resistance of FIG. 17 can beexpressed as a function of voltage V and current i, as follows:

$\begin{matrix}{R = \frac{V}{i}} & {{Eqn}\mspace{14mu}(19)}\end{matrix}$

Similarly, the resistance RH of the hydraulic “circuit” above can beexpressed in terms of pressure head P and volumetric flow rate {dot over(Q)} as:

$\begin{matrix}{R_{H} = \frac{P}{\overset{.}{Q}}} & {{Eqn}\mspace{14mu}(20)}\end{matrix}$

Eqn (20) addresses the resistance of 56 fluid ports of the irrigationablation tip electrode together. However, with the assumption that allports are approximately the same size and therefore the same resistance,the individual resistance of each port may be derived using a parallelresistance network analog, as shown in FIG. 18. The electricalresistance of an individual resistor can then be expressed as

$\begin{matrix}{R_{n} = \frac{56\mspace{20mu} V}{i}} & {{Eqn}\mspace{14mu}(21)}\end{matrix}$

By the same argument, the hydraulic analog may likewise be expressed as:

$\begin{matrix}{R_{H_{n}} = \frac{56\mspace{14mu} P}{\overset{.}{Q}}} & {{Eqn}\mspace{14mu}(22)}\end{matrix}$

Using the above relationship, the hydraulic resistance of a given portgeometry may be quantitatively characterized by measuring the pressurehead P at the inlet to the tip electrode and the result bulk volumetricflow rate {dot over (Q)}.

With reference to FIG. 19, a flow fixture 500 was developed toquantitatively measure hydraulic resistance R_(Hn) for various irrigatedtip shells. The flow fixture 500 includes a thermocouple 501, a waterpressure head 502 (comprising a water tank 503, a pressure gage (ref.verification) 504 and a tip shell 505) and a collection beaker 506.Pressure P within the tip shell 505 is precisely controlled via the headheight. Pressure is related to head height via the following equation:P=ρ(T)gh   Eqn (23)where ρ is water density, g is the local gravitational constant, and his the height of the water column in the water tank 503 above the tipshell 505. Water density ρ is a function of temperature T which ismonitored via the thermocouple on the water tank 503.

Volumetric flow rate {dot over (Q)} is calculated by capturing fluidflow from the tip shell 505 into the beaker 506 over a period of timeΔt. The net mass of the water m_(net) is determined by weighing thefilled collection beaker 506 and subtracting its dry mass. Volumetricflow rate is then calculated as shown below:

$\begin{matrix}{\overset{.}{Q} = \frac{m_{net}}{{\rho(T)}\Delta\; t}} & {{Eqn}\mspace{14mu}(24)}\end{matrix}$

Hydraulic resistance R_(Hn) of an individual port can then be calculatedas:

$\begin{matrix}{R_{H_{n}} = \frac{56\;{\rho^{2}(T)}g\; h\;\Delta\; T}{m_{net}}} & {{Eqn}\mspace{14mu}(25)}\end{matrix}$

Tip shells with various port configurations were first dimensionallycharacterized on a Scanning Electron Microscope (SEM). Results aresummarized in FIG. 20. Following dimensional characterization, eachsample was tested on the flow fixture 500. Volumetric flow rate wasrecorded for each pressure head level setting as shown in FIG. 21. Fromthe linear regressions in FIG. 21, the bulk hydraulic resistance R_(H)for each tip shell can be calculated. Hydraulic resistance R_(Hn) foreach port is therefore equal to 56R_(H), as shown in FIG. 22.

In order to understand the effect of taper angle, the laser drilled portgeometry sample is correlated to the EDM Nominal-Production portgeometry sample as shown in FIG. 23. The correlation plot indicates thatfor the same pressure head, the laser drilled port geometry sample has aslightly lower volumetric flow rate, and is therefore more resistive.However, the areas of the EDM port and the laser drilled port aredifferent, with the laser drilled port having a smaller, morerestrictive area.

Normalizing the data between the EDM sample and the laser sample to anominal 0.0035 inch inlet port diameter (and therefore total area), andthereby eliminating the effect of surface area on port resistanceexposes the hydraulic resistance component due to the 6 degree taperangle, as shown in FIG. 24. In the plot, of FIG. 24, the slope of thecorrelation line is 1.0198, which indicates that the laser drilled porthas increased volumetric flow rate when compared to an EDM tip with thesame port diameter at the same pressure head. The effect of the 6 degreetaper angle is the different between the 1.0198 slope and an idealcorrelation of 1.0.

$\begin{matrix}{{\Delta\%} = {{100 \times \left( \frac{1.0198 - 1.0}{1.0} \right)} = {1.98\%}}} & {{Eqn}\mspace{14mu}(26)}\end{matrix}$Therefore the effect of the 6 degree taper angle is a 1.98% increase involumetric flow rate, and conversely, a 1.98% decrease in hydraulicresistance.

The ranges of hydraulic resistance of a single irrigation EDM onThermoCool SF Irrigated Tip Shell M-5787-03 as tested are shown in theTable of FIG. 25. With referenced to FIG. 26, an exponential fit wasutilized to interpolate for the hydraulic resistance of an EDM tip withan 0.004 inch diameter port based upon the explicitly tested EDMconfigurations of 0.003 inch, 0.0035 inch and 0.005 inch diameter port,respectively. Utilizing the interpolation equations above [which onesspecifically, pls list Eqn (#)], the pressure versus flow relationshipfor the 0.004 inch EDM and laser drilled ports can be shown relative tothe validated ranges, as shown in FIGS. 27 and 28. By reducing thepressure versus flow sensitivity by 2% to account for the 6 degree taperangle of the laser drill process, the upper specification limit USL forthe proposed laser port can also be illustrated relative to thevalidated range. Based on these graphical representations, the laserdrilled tip shells with 6 degree taper angle and equivalent inlet portdiameters perform with the validated hydraulic resistance envelope forthe original straight port EDM catheter.

It is understood that the present invention includes any irrigatedablation tip electrode where any or all of the above ratios are met.That is, an irrigated tip electrode, whether or not it has a two-piececonfiguration, provides the advantageous features of the presentinvention where its relevant dimensions and parameters enable the tipelectrode to satisfy any or all of the above ratios. The precedingdescription has been presented with reference to certain exemplaryembodiments of the invention. Workers skilled in the art and technologyto which this invention pertains will appreciate that alterations andchanges to the described structure may be practiced without meaningfullydeparting from the principal, spirit and scope of this invention. It isunderstood that the drawings are not necessarily to scale. Accordingly,the foregoing description should not be read as pertaining only to theprecise structures described and illustrated in the accompanyingdrawings. Rather, it should be read as consistent with and as supportfor the following claims which are to have their fullest and fairestscope.

What is claimed is:
 1. An irrigated ablation catheter, comprising: anelongated catheter body; a deflectable section distal to the catheterbody; a tip electrode distal to the deflectable section, the tipelectrode comprising: an outer shell defining a cavity, the shell havinga predetermined plurality of fluid ports, each fluid port contributingto a total fluid output area of the tip electrode; an internal memberincluding a fluid inlet into the tip electrode, the fluid inlet having afluid input area; wherein each fluid port is tapered, and has a fluidport aspect ratio of a thickness of the outer shell to a diameter of thefluid port of less than 3.25.
 2. The catheter of claim 1, wherein thediameter of each fluid port at an inlet to the fluid port is betweenabout 0.003 inch and 0.005 inch.
 3. The catheter of claim 1, wherein thediameter of each fluid port at an inlet to the fluid port is betweenabout 0.003 inch and 0.004 inch.
 4. The catheter of claim 1, whereineach fluid port is tapered by an angle between about 0 and 6 degrees. 5.The catheter of claim 1, wherein the outer shell has a shell wallthickness between about 0.003 inch and 0.004 inch.
 6. A catheter ofclaim 1, wherein, the predetermined plurality of fluid ports is about56.
 7. The catheter of claim 1, wherein each fluid port is tapered by anangle between about 4 and 6 degrees.
 8. An irrigated ablation catheter,comprising: an elongated catheter body; a deflectable section distal tothe catheter body; a tip electrode distal to the deflectable section,the tip electrode comprising an outer shell defining a cavity and havinga predetermined plurality of fluid ports, each fluid port contributingto a total fluid output area of the tip electrode, the tip electrodealso having a fluid inlet with a fluid input area; wherein the tipelectrode has a diffusion ratio less than about 1.8, and each fluid porthas a tapered configuration with a taper angle between about 4 and 6degrees.
 9. The irrigated ablation catheter of claim 8, wherein thetapered configuration includes a frustoconical configuration.
 10. Theirrigated ablation catheter of claim 8, wherein each fluid port has afluid port aspect ratio of a thickness of the outer shell to a diameterof the fluid port of less than 3.25.
 11. The irrigated ablation catheterof claim 8, wherein each fluid port has an inlet diameter rangingbetween about 0.003 inch and 0.005 inch.
 12. The irrigated ablationcatheter of claim 8, wherein the outer shell of the tip electrode has ashell thickness ranging between about 0.003 inch and 0.004 inch.
 13. Anirrigated ablation catheter, comprising: an elongated catheter body; adeflectable section distal to the catheter body; a tip electrode distalto the deflectable section, the tip electrode comprising: an outer shelldefining a cavity, the shell having a predetermined plurality of fluidports, each fluid port contributing to a total fluid output area of thetip electrode; an internal member including a fluid inlet into the tipelectrode, the fluid inlet having a fluid input area; wherein the tipelectrode has a predetermined diffusion ratio, a predetermined fluidport ratio and a predetermined inlet aspect ratio, and the cavity has avariable inner cross section, wherein the cavity has an innercross-section that varies along a length of the tip electrode, andwherein each fluid port has a tapered configuration and a fluid portaspect ratio of a thickness of the outer shell to a diameter of thefluid port of less than 3.25.
 14. The catheter of claim 13, wherein eachfluid port is tapered by an angle between about 4 and 6 degrees.